1. Field of the Invention
The present invention relates to user-programmable circuits. More particularly, the present invention relates to antifuse-based user-programmable circuit architectures and to a multi-level antifuse interconnection architecture.
2. The Prior Art
In numerous user-configurable architectures employing antifuses, a plurality of antifuses may be provided to potentially connect a given signal line or node to other signal lines or circuit nodes. While providing numerous antifuses on a single conductor enhances the connectability of that conductor and, hence, the versatility of the user-programmable architecture in which the antifuses are employed, a tradeoff is presented between versatility and performance problems and degradation. First, all antifuses common to a signal line which remain unprogrammed in a final configured circuit act as individual capacitors which each contribute capacitance to the total capacitance of the signal line. In addition, antifuses which employ an antifuse material such as amorphous silicon are characterized by some degree of leakage in their unprogrammed state. Depending on the voltage potentials to which the other ends of these antifuses are connected, a random leakage current will be present at nodes, such as conductors, to which more than one unprogrammed antifuse is connected. Finally, experience has shown that unprogrammed antifuses may be subjected to voltage stresses during programming of other antifuses, creating reliability issues for the circuits in which they reside.
A typical example of such antifuse distribution is an interconnect structure including a first conductor running in a first direction which intersects a plurality of second conductors running in a second direction. In order to maximize connectability in a user-programmable interconnect architecture, antifuses are provided at each intersection between the first conductor and one of the second conductors. Programming of all of these antifuses is rare in a given circuit implemented by this architecture. It is plain that the capacitance of the first conductor is significantly affected by the presence of the unprogrammed antifuses which are connected to it, as is the current leakage through that conductor contributed to by each unprogrammed antifuse. Finally, the voltages which must be placed across selected antifuses disposed between the first conductor and selected ones of the second conductors to program them may have an undesired stressing effect on adjoining antifuses which are supposed to remain unprogrammed.